Telephone conferencing is the process of connecting three or more parties so that a three-way telephone conversation can be conducted. There are many different conferencing algorithms. One of the simplest algorithms to implement in an analog switch entails simply bridging all parties on a common talk path. This essentially creates one big party line that works well in conventional analog switches for three or four, or maybe even five, parties. Above five parties, serious transmission degradation occurs because of line impedance variations. Circuit loading and signal hogging caused by these line impedance variations can result in serious transmission degradation. Various algorithms based on selecting the most likely party to talk and forcing all other parties to listen have been used, but all suffer from the common problem that the talker chosen by the conference circuit is not always correct. In larger conferences, with a mixture of off premise stations and outside parties, especially where specialized common carrier facilities are used, it is very likely that the outside parties will never be able to talk.
With digital switches, direct bridging is not possible, but can literally be simulated by summing the samples of all parties and transmitting the result to all. This simulation is extremely easy to implement, but results in a sidetone level of zero decibels because each party gets back its own sample, as part of a grand total, at the same level that it went out. This problem can be solved by subtracting each party's sample from the grand total before sending the grand total back to each party. Unfortunately, this increases the complexity of the circuitry required, since there are as many different outputs as inputs.
The modified algorithm works well for three parties; with four or more parties, two of the inherent properties of digital systems cause difficulty. The first difficulty is caused by the four wire nature of digital switches. Substantially all telephone instruments and most trunk facilities are two-wire devices, simultaneously transmitting and receiving on a single pair of wires. Since the switch itself is a four-wire device, with separate transmit and receive paths, a hybrid conversion circuit is used to separate the two directions at the two-wire interface. The hybrid conversion circuit is always imperfectly balanced and results in reflection of some of the incident energy. This reflected energy may be attenuated as little as twelve to fourteen decibels, meaning the transmit sample from a port may contain a voltage which is about a quarter of that sent to the port as a receive sample.
This twelve to fourteen decibels of attenuation is more than adequate for normal two party conversations. In large conferences, however, each party reflects energy which is summed. With five parties in a conference, each reflecting a quarter of the incident voltage, the summation of these reflections is equal to the contribution of the speaking party. Besides creating an undesirable sidetone level of zero decibels, depending on the reflection phase angles and path delays, significant positive feedback can result causing undesirable hollowness, ringing or even oscillation.
Another problem is related to the hard-limiting characteristic of .mu. law PCM systems. The dynamic range of the .mu. companding law has been carefully optimized for two party conversations considering the statistical properties of voice signals. Since separate voice signals, such as those of the various parties in a large conference, are uncorrelated, except for the component due to reflection, doubling the number of simultaneous potential talkers raises the total average voice signal power by about three decibels. The peak signal voltage however, can theoretically increase by about six decibels since the uncorrelated nature of the signals does not affect the possibility that all parties can be, for any given sampling instant, producing maximum code words. Thus, a four party conference can have an average power six decibels, or four times that of a single speaker, with voltage peaks twelve decibels higher.
An eight party conference can have an average power nine decibels higher with 18 decibel higher peaks. This has three consequences. First, the number of bits used to represent the grand total must be large enough to prevent overflow and rollover. Alternatively, logic must be provided to detect overflow and replace the sum up with a maximum level code word of appropriate polarity. Second, the output linear to .mu. law conversion must be able to handle larger code words than is specified for .mu. law and convert them to the appropriate maximum level .mu. law words. The third consequence is that the probability of clipping is significantly increased over that for a single party. Clipping probability is not multiplied by the number of parties for a number of reasons, the most important being psychoacoustic. Participants in a conference generally take turns, and even during contention, not all conferees participate. It will be appreciated that it would be highly desirable to have an electronic telephone system that makes effective telephone conferences possible.
In prior conferencing bridges, a PROM was used at the output of the arithmetic unit to handle overflow and zero substitution. The PROM introduced undesirable delay. To compensate for the delay caused by the PROM, faster, more complex circuitry was used. Unfortunately, the faster circuitry is expensive. It will be appreciated that it would be highly desirable to have a conferencing bridge that eliminates the speed penalty of the PROM.